Officials said “unscrupulous actors” have been “marketing fraudulent test kits and using the pandemic as an opportunity to take advantage of Americans’ anxiety.”

The FDA action came after the agency was criticized for a March 16 policy that allowed commercial test makers to sell antibody tests after validating their own data and notifying the FDA. The result, some testing experts said, was a flood of products of dubious quality that confused hospitals, doctors and consumers — “a wild, wild West” environment, said Scott Becker, chief executive officer of the Association of Public Health Laboratories, which represents state and local public laboratories. He and others in the laboratory field urged the agency to exert its authority over the tests.

Antibody, or serology, tests are designed to identify people who may have overcome covid-19, the disease caused by the novel coronavirus, including those who had no symptoms, and developed an immune response. They are not designed to detect active infections. Some officials tout the blood tests as a way to identify people who have developed immunity and can safely return to work. But many scientists say evidence is lacking that the tests can prove someone has immunity from the coronavirus or for how long.

FDA Commissioner Stephen Hahn emphasized those uncertainties in a Monday call with reporters. “Whether a test should be a “ticket for someone to go back to work as the sole item, my opinion on that would be no, because there are a lot of unanswered questions,” he said.

Inaccurate tests can have serious consequences, experts warn. Incorrect results could indicate people have immunity against the virus when they don’t, potentially resulting in behavior that would endanger them and those around them. Such issues are taking on special importance as federal and state officials debate strategies, including using serological testing, to help determine when they can end state and local lockdowns. Antibody tests also are considered a way to determine whether people who have recovered from covid-19 can donate blood that can be used to manufacture “convalescent plasma,” which is being tried as a potential treatment.

Under the new rules, companies already marketing tests will be required to file applications for FDA emergency-use authorizations, including data on their tests, within 10 business days. Makers of new tests will have to file such applications within 10 days of notifying the agency of their plans to go to market. Under such authorizations, the FDA allows unapproved medical products to be used in a public health emergency without being subjected to a full-fledged review.

FDA officials said their previous policy was appropriate because antibody tests are not used to diagnose covid-19 and that allowing the tests on the market early provided information about the prevalence of covid-19 in some populations and whether antibodies confer immunity to the virus.

“However, flexibility never meant we would allow fraud,” Anand Shah, deputy FDA commissioner for medical and scientific affairs, and Jeff Shuren, director of the agency’s Center for Devices and Radiological Health, said in a statement. “We unfortunately see unscrupulous actors marketing fraudulent test kits and using the pandemic as an opportunity to take advantage of Americans’ anxiety.”

They noted some test makers have wrongly said their products have been approved or authorized by the FDA or could be used to diagnose covid-19. In addition, they said, independent evaluations by the National Cancer Institute have showed some performed poorly. The agency is stopping some tests at the border. Many of the imported tests are from China.

The agency said it knows of about 160 serology tests being marketed in the United States that have not received authorization; of those, about two dozen are laboratory-developed tests that must be performed in special laboratories certified to do high-complexity testing. Those tests, many of which are being used at academic medical centers, won’t be required to have emergency-use authorizations, although the agency said it encourages it.

Overall, the agency has authorized 12 antibody tests for emergency use, including one by Roche. The FDA said it is working with companies on authorizations for an additional 250 serology tests and is taking steps to streamline the process. If commercial test makers don’t submit applications for authorizations within the 10-day time limit, the agency plans to make that information public and consider enforcement action to take the products off the market.

The FDA also said it expects all antibody tests — those made by both commercial manufacturers and laboratories — to meet certain standards on sensitivity (the tests’ ability to detect antibodies) and specificity (the ability to distinguish people who are infected from those who are not).

The agency received praise for the change. “The Food and Drug Administration made the right decision by walking back its policy on serology testing for covid-19,” said Becker of the public health labs group. “We’ve long been concerned that allowing tests on the market that have not been approved and authorized for use is a recipe for disaster. This revised policy makes a lot of sense and should have been in place over the last six weeks.”

The FDA’s initial hands-off policy also drew criticism from some congressional Democrats. Rep. Raja Krishnamoorthi (D-Ill.), chairman of the House Oversight Committee’s economic and consumer policy subcommittee, recently conducted an investigation of the policy and concluded it had allowed scores of flawed tests onto the market.

As part of its program to assess the quality of antibody tests, the NCI has validated 13 kits, according to FDA officials who said they will make the results available after reviewing them.

Even high-quality tests have limits and can result in errors, experts warn. An antibody test used in areas without many cases of covid-19 may produce as many or more false results as true results because the likelihood of finding someone who has been infected is very small. In those situations, it may be necessary for some people to have two antibody tests to get reliable results, the FDA said.